Fault current limiters serve to prevent unacceptable large current surges in modern power systems, in particular high power systems, due to any fault event such as short circuits etc and, thus, protect the power systems from damage.
Superconductors, in particular ceramic oxide high temperatures superconductors, offer a great potential as fault current limiters which enable rapid and effective current limitation, automatic recovery, negligible impedance during normal operation and application at high voltage.
Current limiters based on high temperature superconductor materials make use of the property of such superconductor materials to switch from their superconducting state with essentially no resistance at normal operation conditions to a resistive state if at least one of its critical temperature (Tc), critical magnetic field (Hc) or critical current (Ic) is exceeded.
For example, in case of fault the current flowing through a superconductor material exceeds the critical current of the superconductor material due to large surge current and the superconducting material undergoes transition from the superconducting to the non-superconducting state with resistance. This transition is also termed “quenching”.
However, in practice, the material of superconductor bodies shows inhomogeneities causing non-uniform quenching wherein some regions of the superconductor body becomes resistive before other regions of the body. In this case the already quenched part of the superconductor body is overheated and may burn out leading to destruction of the superconductor body.
For solving the problem of local overheating and for obtaining a fast and uniform transition of the superconductor to the non-superconducting resistive state it is known to provide the superconductor body with a shunt of normal conductive material the resistance of which being lower than the resistance of the superconducting material in its normal conducting state. In case of a sudden temperature increase during quench the current is bypassed to the shunt and heat is dissipated from the superconductor body.
Furthermore, for avoiding local burn out triggering techniques are known promoting fast and uniform transition of the superconductor material from its superconducting state to the non-superconducting state.
One of these techniques makes use of the fact that the critical current density of a given superconductor material decreases if an applied magnetic field increases. According to this triggering technique an external magnetic field is applied to the superconductor body in case of fault event. By that magnetic field the critical current density is reduced which, in turn, promotes quenching.
A superconductor component particularly suitable as fault current limiter with magnetic field assisted quenching is disclosed in EP 1 524 748 A, which is included herein by reference.
The superconductor component of said patent application comprises a superconductor body of low inductive shape such as a rod, tube or plate, preferably a rod or tube with essentially round cross-sectional geometry. Around that superconductor body a coil is wound being made of a normal conductive material, such as a metal. That coil is electrically connected in parallel to the superconductor body wherein the ends of the coil are usually fixed to the respective end sections of the superconductor body.
In fault event, i.e. when the critical current is exceeded, the superconductor material starts to quench. Resistance and corresponding voltage (flux flow) are built up which causes that a part of the current is bypassed automatically to the parallel connected coil without any external control.
Due to the current now flowing through the coil a magnetic field is built up which, in turn, reduces the critical current density of the superconductor material. In the consequence the fault current limiter of this patent application has a so-called self-triggering design triggering the quench automatically without external control.
In practice a plurality of such superconductor components is connected in series for forming a fault current limiter.
In order to connect the fault current limiter with a power system or with each other electrical contacts are provided at the ends of the superconductor component.
As is clear from the above such superconductor components have to withstand high mechanical, thermal and magnetic forces especially during fault events.
For example, it has been observed that in known high temperature fault current limiters of a design as set out above with a superconductor body with a shunt coil wound around its outside diameter the superconductor body is liable to be broken or weakened in the region of connection of the superconductor body with the electrical contacts, that is at the end section.
It is this end section where the coil ends. That is, the electrical contacts are located at the region of the ends of the coil.
Considering the shape of the magnetic field of a current carrying coil this field is uniform at the middle part of the coil whereas at the coil ends, in particular at the exit site of the magnetic field from the coil interior, the field becomes non-uniform and especially has components parallel to the radius (“radial components”).
As set out in more detail in the following, it is believed by the present inventor that the reason for the observed damages are forces generated by currents induced in the electrical contact by the magnetic field and the interaction of these currents with the radial components of the magnetic field.
For the present invention the term “non-uniformity” of the magnetic field relates to the radial components of this field.